Known in the art is a process and apparatus for sensing spatial coordinates of a point on an article (cf., for example, Japanese Pat. No. 52-7938 NPC 106 c 34) wherein the process for sensing spatial coordinates of points of an article starts with forming a main coherent light field which is a .delta.-like spatial illumination distribution. The uniform time characteristic of this .delta.-like illumination distribution has been taken as the optical characteristic of the light field, effecting the one-to-one correspondence between the magnitude of the optical characteristic in the field point which coincides with the selected point of the article and the coordinate of this point. The uniform distribution is attained by uniform scanning of the selected point on said article by a narrow laser beam. The magnitude of the optical characteristic is then measured in this particular point of the main coherent light field. The magnitude of the optical characteristic is the time interval between the moment when the narrow laser beam meets the selected point of the article and the moment when scanning has begun. This time interval is the function of the position of the selected article point. The measured interval of time helps to find the coordinates of the selected point of the article in the direction of scanning. If the linear velocity is V, the coordinate x of the selected point of the article is equal to EQU x=V.T, (1)
where T is the measured time interval between the beginning of scanning and the moment when the narrow laser beam meets the selected article point.
The known apparatus for sensing spatial coordinates of a selected point of an article comprises transmitting and receiving parts. The transmitting part is composed of the main transmitting optical channel which has a laser in order to produce a main coherent light field in the space near the article, an optical characteristic setting unit which is a unit scanning the selected article point by the laser beam in the direction of measurement, and an optical converter which forms a narrow laser beam. The receiving part of the apparatus comprises a main receiving channel having a photodetector aligned with said point of the article, and a measuring unit coupled to a coordinate measuring unit. In addition, the receiving part has a reference signal transmitter generating a scanning start signal. The photodetector is optically tied in with the selected point of the article by placing the photodetector directly in the selected point of the article.
The error of measuring the coordinate of a point by means of the proposed method and apparatus is basically dictated by the stability of the scanning velocity V and errors in defining the time interval T, which in principle cannot be sufficiently reduced due to the existing divergence of the laser beam.
Known in the art is a method for sensing spatial coordinates of a point of an article and an apparatus therefor (cf., for example, British Pat. No. 1,521,351, Cl. G 01 B 11/14, B 11/24, filed Jan. 19, 1976) wherein a coherent light field is formed on the surface of the article along one of the coordinate axes near the surface of the object, this light field being an array of uniformly spaced light planes. One of the optical characteristics of said light field is distribution of the phase thereof along the direction of measuring the coordinate of the article point. As the light field moves in relation to the article, the phase of this light field is continuously measured as from some initial moment of time. The measured phase helps to define the coordinate of the article point in relation to the element of the article where the light field phase has been measured at the initial moment.
The known apparatus for sensing spatial coordinates of an element of an article comprises transmitting and receiving parts. The transmitting part comprises a main transmitting optical channel which has a laser to produce a main coherent light field near the article, an optical characteristic setting unit which is a beam splitter, and an optical converter which is an objective lens. The receiving part of the apparatus comprises a photodetector optically aligned with the selected point of the article, and a measuring unit coupled to a coordinate measuring unit.
But these known methods and apparatus cannot measure the coordinates of a selected point of the article at any moment because, according to the invention, the article should be continuously scanned by the light field, starting from some initial point. Accidental interruptions of the measuring signal contribute to accumulation of the error in the coordinate of the selected point of the article. Such accidental interruptions of the measuring signal are accounted for by the static properties of light scattered by the surface of the article. Moreover, the above method and apparatus cannot directly measure the coordinates of a selected point of an article in relation to a point outside the article.
The closest prototype of the invention disclosed herein is the known method for sensing magnitude and direction of lateral displacement (cf., for example, U.S. Pat. No. 3,930,734, M Cl. G 01 B 9/02, Apr. 26, 1974) wherein in order to measure spatial coordinates of a point of an article along one of coordinate axes the following steps are taken:
a main coherent light field is produced in the area of the article, whose optical characteristic is a uniformly distributed phase through 0.degree.-360.degree. in the direction of measurement of the coordinates obtained by interference of two converging coherent light beams,
the phase of the coherent field is determined in the point of the field coinciding with the selected element of the article,
the coordinate of the selected point of the article is sensed by the measured phase magnitude in the direction of the coordinate measurement.
However, in this case to accurately measure the coordinates is a task involving a complicated technological operation of precision measurement of the light field phase. Thus, for example, if the article is 1 meter long in the direction of coordinate measurement, to measure the absolute coordinate of a point of the article with an absolute error of 10.sup.-4 m the phase of the coherent light field should be measured with an error of 0.036.degree., which is quite a difficult technological problem. The task becomes utterly impracticable if the coordinate of an element of the article is to be measured by the above method with an error comparable with the light wave length.